In a typical communication system, analog (or digital) signals are transmitted from a transmitter to a receiver through a transmission channel. For example, the transmitted signal can include a differential signal transmitted through a twisted pair of transmissions lines. The transmission lines are coupled to a receiver that is fabricated on an integrated circuit for decoding the transmitted signals into single-ended or complementary analog or digital output signals.
In the design of high performance receivers, it would be advantageous if the fastest, smallest transistors available for the technology in which the integrated circuit is fabricated could be used. This would achieve the highest switching speed while consuming the least area and the least power. However, the fastest transistors that are available in a given technology are often low-voltage transistors, which may have voltage ratings that are lower than the power supply for the integrated circuit. If the available power supply has a higher output voltage than the voltage rating of the transistors used to form the receiver, the most common solution is to use a voltage regulator for reducing the original power supply output voltage to a lower voltage that can be used for powering the receiver. However as power supply voltages and transistor voltage ratings continue to reduce, it becomes very difficult and complex to build full voltage regulators that can regulate voltages accurately at these small levels within sufficient tolerances.
For example if the original power supply voltage, VDD, has a tolerance of ±10%, then the reduced voltage produced by the voltage regulator would also have a tolerance of ±10, plus whatever inaccuracies are introduced by the voltage regulator. These inaccuracies can result from operational amplifier offset voltages and resistor mismatches, for example. A reasonable expectation would be that the reduced voltage at the output of the regulator would have a tolerance of about ±12% of the target voltage. This tolerance is quite large and can pose problems for low power supply voltages that are just above the sum of the N-channel and P-channel threshold voltages of the transistors in the receiver.
Improved regulators are therefore desired for biasing high speed, low-voltage receivers so as to reduce the overall range of bias currents in the receiver and provide tighter electrical response at the output of the receiver.